You can use pretty much everything - in the end it all depends on your realworld haggle skills
. There might be some NPCs that only accept the local coins, but since all players are from a gazillion different "Larp countries" and will go back there after the event and travel through a dozend other "in their character life".... they tend to accept anything that has a nice look and feel and is not visibly a real-world coin of an existing country.
There is no official interchange course or even an ingame talent for your character for this... if you, the player are good in bargaining in character... you can make some nice deals
. Oh... and one thing:
People LOVE copper coins as they are so far more useful than silver or gold. With coppers, you can pay beggars, give a tip, pay for a cup of tea or so. Giving silver and expecting change... well. People do not like giving change
.